This invention relates in general to extrusion and, more specifically, to novel apparatus and processes for fabricating seamless tubes including thin-walled, large diameter tubes.
In the art of extruding thin-walled, large diameter polymer tubes from an annular orifice, air is introduced into the hot tubing as it emerges from the extruder to prevent collapse of the tubing walls while the thermoplastic material is cooling and solidifying. The molten polymer tube, if unsupported, tends to sag and deform under its own weight with bonding occurring at any point of contact with itself or with adjacent surfaces of the extrusion apparatus. In the blown film extrusion technique, the tube is also expanded by the internal air pressure. The formed tubing is thereafter recovered by passing it between pinch rollers which flatten the tubing. The flattened tubing may subsequently be transported to a suitable device for rolling the flattened tubing for further processing in a device such as a bag making machine.
Although thermoplastic film tubing has been manufactured commercially, difficulties have been experienced in preparing thin-walled, large-diameter, pliable tubes having precise dimensional uniformity in wall thickness, tube diameter and conicity which would satisfy high tolerance requirements. Techniques for forming thermoplastic tubes for garbage bags such as by blown extrusion are generally inadequate for devices that must meet high dimensional standards. For exmaple, electrostatographic members such as photoreceptor substrates for use in sophisticated high speed electrophotographic copiers, duplicators and printers must have a uniform thickness and diameter so that the photoconductive layers thereon are maintained at precise distances from optical exposure devices, charging devices, cleaning devices and image transfer stations. Thus, these distances must be uniform across the width and along the circumference of the photoreceptor belt and be free of any imperfections such as creases. In addition, high speed electrophotographic copiers, duplicators and printers require belts having a uniform conicity, i.e. uniformity of diameter from one side of the belt to the other, to insure proper belt support and tracking, particularly during extended high speed runs. Moreover, reproducibility of the diameter of a photoreceptor belt is necessary to insure proper fit on the belt support drive systems of modern electrophotographic copiers, duplicators and printers.
In the blown film extrusion process, the type of pinch rollers utilized to maintain the pressure of the internal air in the blown tube causes two diametrically opposite permanent creases to form in the tubing running parallel to the tube axis. These permanent creases render blown film extrusion belts unsuitable for modern high speed electrophotographic imaging systems because the creses interfere with charging, exposure, development, transfer and cleaning.
Various techniques utilizing internal and external sizing mechanisms have been developed for producing tubular films. For example, an internal mandrel process and apparatus for the production of oriented tubular film is disclosed in U.S. Pat. No. 4,062,916 to Skilling in which rupture of the extruded tube by the pressure of inflating gas is prevented by means of a cup seal located between the mandrel and inflating zone, the cup seal being in peripheral engagement with the internal surface of the tube and presenting a substantially concave surface on the mandrel. The requirement for a cup seal and a protruding lip on the seal totally prevents leakage past the seal. In order to properly function as a seal, the seal must be resilient. Moreover, the seal does not perform a sizing function as seen, for example, In FIG. 1 of U.S. Pat. No. 4,062,916 where the extruder tube is relatively thick at the seal and is subsequently blown into a large diameter thin-walled tube elsewhere down the production line.
In U.S. Pat. No. 3,818,790 to North et al, a thermoplastic resin is extruded through a ring die to form a tubular film when the film is partially cooled by means of a cooling gas before the tube is drawn over a cooling mandrel, the improvement being the use of multi-stepped adjustable rings to direct a cooling gas onto the external surface of the film between the die gap and mandrel, thus defining a series of orifices between the rings and the film to effect a gradient in pressure from the die to the mandrel. Static air pressure is employed on both sides of the mandrel to inflate the film bubble.
In Japanese Patent Application No. 1974-122,555 to Showa Denko, filed Sept. 26, 1979, an internal stabilizing device forming film bubbles is disclosed in which a number of disks are arranged coaxially on a solid shaft. The extruded film is drawn inwardly toward the spaces between adjacent disks supported on a solid rod. Thus, the contact pressure of the tubular film against the multiple disks is increased to stabilize the film. The tubular film is thereafter formed into a large diameter film bubble. According to this patent application, a single disk unit by itself is ineffective.
Another technique for forming thermoplastic tubing is disclosed in U.S. Pat. No. 2,519,375 to Jargstorff et al. The technique disclosed involves an external cooling die and does not employ a sizing disk or hollow mandrel. A stagnant gas pocket is utilized to expand the hot plastic tubing against the internal surface of the cooling die. The tube is bent to permit the formation of a liquid seal to maintain the stagnant gas pocket.
An external cooling ring is also described in U.S. Pat. No. 4,341,729 to Hayashi et al. The external cooling ring is used as a suction ring which simultaneously sucks air used for heating the stretched upstream section of a tubular film and air used for cooling the downstream section of the tubular film. No sizing disk or hollow mandrel are disclosed.
In Plastic Extrusion Technology and Theory, G. Schenkel, Transl. by L. E. H. Eastman, B. S. Clyde, Ed., Lliffe Books Ltd., London, American Elsevier, New York, p. 322, apparatus is shown in which a thin-walled tube is extruded over a water-cooled sizing disk. It appears that air is channeled through the die and sizing disk and introduced downstream of the sizing disk in FIG. 11.2 (b). This introduction of air downstream through the sizing disk renders difficult adequate control of the pressure in the chamber between the die and sizing disk.
Accordingly, there continues to be a need for an improved and more effective system for forming seamless, thin-walled, creaseless belts having wall thicknesses and tube diameters which satisfy high tolerance standards, particularly for tubes meeting the demanding dimensional uniformity requirements of modern, precision, high speed electrostatographic imaging systems.